monte-en-l'air

cat burglar

“— Si c’était un monte-en-l’air, il serait passé par le toit et il serait descendu en rappel : la serrure du toit aurait été forcée.” — "If it was a cat burglar, he would have got in through the roof and abseiled down; the lock on the roof would have been forced."

From monter en l'air (“to go up in the air”), from the fact that a burglar may have to climb over walls to break into a house.